Bug #2264: Use fixed variable name for perfdata of df check

Date: 2014-12-08State: openClass: nastinessComponent: checks

Currently it uses the mount point as variable name. This is nonsense and leadsto problems. In one case we even get too long filenames for the resulting RRDsin MULTIPLE mode.Resolution:- Change to simple variable name- Create some deprecated global option that simulates the old behaviour for some time.